What are the vulnerabilities of PPTP VPN

It is said often enough that PPTP VPN is insecure and you shouldn't use it.

In what ways *specifically* is it insecure? What are the implications of using it?

Can an attacker gain unauthorized access to the network by logging in to the VPN? Can user account passwords be compromised?

Or is it just that the data passing over the VPN can be decrypted / manipulated?
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Frosty555Asked:
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Wayne88Commented:
"Don’t use PPTP. Point-to-point tunneling protocol is a common protocol because it’s been implemented in Windows in various forms since Windows 95.  PPTP has many known security issues, and it’s likely the NSA (and probably other intelligence agencies) are decrypting these supposedly “secure” connections. That means attackers and more repressive governments would have an easier way to compromise these connections."

Which is the Best VPN Protocol? PPTP vs. OpenVPN vs. L2TP/IPsec vs. SSTP

http://www.h-online.com/security/features/A-death-blow-for-PPTP-1716768.html
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andreasSystem AdminCommented:
PPTP is dead. Dont use it not just agencies can crack. Everybody can do for as less as 200US$ worth of computing power from the cloud.

If you use PPTP you write an invitation card to attackers.

http://www.h-online.com/security/features/A-death-blow-for-PPTP-1716768.html

Once again. its dead do not use it anymore. Its broken beyond any repair.
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asavenerCommented:
It is said often enough that PPTP VPN is insecure and you shouldn't use it.

In what ways *specifically* is it insecure? What are the implications of using it?

Can an attacker gain unauthorized access to the network by logging in to the VPN? Can user account passwords be compromised?
PPTP can handle authentication in different ways.  Typically, these are weaker authentication methods which can be cracked and/or replayed.  The exact mechanism depends on the authentication method, but I would consider it likely that a username/password could be compromised, and thus allow an attacker remote access to your network.

Or is it just that the data passing over the VPN can be decrypted / manipulated?
This, too.

The goal should be data integrity, data accessibility, and data confidentiality, (and increasingly, non-reputability).  A VPN's purpose is integrity and confidentiality.  PPTP fails on both of these.
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